There has been known an optical granular object sorting machine which blows away granular objects, such as a grain of cereal or a resin pellet, with an air jet to sort good pieces from defective pieces and removes a foreign object or the like mixed in granular objects with an air jet.
A granular object sorting machine of this type blows away a granular object dropping along a predetermined path from an end portion of a conveyance passage with an air jet based on a signal indicating detection of a defective piece or the like, thereby sorting out the granular object.
The above-described granular object sorting machine blows away a defective piece or the like among granular objects dropping in succession in large amounts with a jet of air. In order to blow away only a defective piece or the like in question with high accuracy without involving other granular objects, the granular object sorting machine needs to provide an air-jet nozzle with a valve with high responsiveness.
For this reason, there is proposed a piezoelectric air valve which can open and close a valve at high speed through use of a piezoelectric element (see Patent Literature 1).
The piezoelectric air valve disclosed in Patent Literature 1 takes advantage of properties of a piezoelectric element excellent in high-speed response performance. The piezoelectric air valve includes a displacement magnification mechanism for magnifying a small displacement of a piezoelectric through leverage.
In the piezoelectric air valve, when a voltage is applied to a piezoelectric element, a displacement in a direction of elongation of the piezoelectric element is transmitted to a valve element via the displacement magnification mechanism. The valve element is quickly moved to open the valve.
In the piezoelectric air valve, when the voltage application to the piezoelectric element is terminated, restoring force associated with restoration to normal state of the piezoelectric element is transmitted to the valve element via the displacement magnification mechanism. The valve element is quickly brought into contact with a valve seat to close the valve.
An optical granular object sorting machine which includes an air-jet nozzle using the above-described piezoelectric air valve blows away a defective piece or the like with high accuracy and is at low risk of involving good pieces or the like in front and behind the defective piece or the like because the piezoelectric air valve is more excellent in responsiveness at the time of valve opening/closing than a conventional electromagnetic valve.
Many optical granular object sorting machines include an air-jet nozzle using a multi-valve unit composed of a plurality of valves provided in parallel to efficiently sort out granular objects.
To construct a multiple air valve using the piezoelectric air valve disclosed in Patent Literature 1, unitary air valves, the number of which is equal to the number of valves required, need to be coupled. Assembly work of the multiple air valve thus takes a long time.